Abstract

Due to mounting evidence that the early environment experienced by a juvenile can affect the morphology and physiology of the adult, there is currently great interest in how environmental variability may shape the behavioural phenotype, and whether such shaping has adaptive benefits. It is clear for example that the developmental environment will have immediate effects on an animal in terms of its survival and performance. Individuals with access to little food or exposed to high levels of predation will have lower survival, and resource-poor surroundings may mean that a young individual is unable to forage successfully, or disperse as normal. However, there is now increasing evidence to suggest that early environmental conditions are also important in determining the success of the adult, meaning that experiences during early development can have significant long-term effects.
In this thesis, I consider the effects of diet quality and exposure to stress in postnatal life on behavioural traits in adulthood, using the zebra finch (Taeniopygia guttata) as a model species. There is already evidence to suggest that the early environment can shape behaviour, and consequently, many aspects of behaviour may have important developmental origins that are not a direct result of the genotype.
Two populations of birds were used throughout this collection of studies. The first was raised under different diet regimes, in which both diet quality and consistency were manipulated during the first weeks of life. The second did not undergo any diet manipulation, but were given oral doses of the stress hormone corticosterone or a peanut oil control in the nestling phase. Both populations then underwent a variety of behavioural tests to determine the effects of their early life experiences.
In examining the behaviour of zebra finches, I found that these birds displayed distinct and repeatable behavioural traits, and discuss the idea that they may even exhibit behavioural syndromes, since I was able to show strong correlations between various responses. I studied the effect of both exposure to varying diet quality and consistency in early life on various behavioural traits in adulthood, and found evidence to suggest that certain behavioural responses are linked to the consistency of the nutritional environment experienced by the chick. When tested in adulthood, birds that had experienced a consistent early environment (regardless of food quality) showed a strong trend towards being bolder than birds raised on a variable diet, and this result was replicated in a subset of the original population over a year later, suggesting real longitudinal effects. Such an outcome may suggest that certain behavioural traits are developed in response to environmental sampling, so as to maximise fitness in the anticipated environment. Similarly, I investigated the behaviour of birds exposed to corticosterone in early life, but found no evidence to suggest that their behavioural traits had been directly shaped by exposure to this hormone. Interestingly however, relative growth rate was linked to individual boldness, a finding which could again possibly be explained through adaptive environmental shaping, in which growth rate is used as a proxy for environmental richness.
Using subsets of the populations of birds that were examined for personality traits, I also considered the effects of the early environment on adult performance in two simple memory tests; specifically studying the ability of birds to search for and remember the location of a food item using environmental cues. In both the diet and hormone manipulated groups, I was able to show that those individuals that had experienced a sub-optimal early environment were compromised in their performance on these simple learning and memory tasks. Again, these were long term effects, since the tests were carried out many months after the birds became adult, suggesting they could not be compensated for, and consequently, since spatial and visual memory are important throughout life, could not be said to be adaptive under any circumstances.
Finally, I also looked for effects of the diet and hormone treatments on the ability of the birds to become dominant. In tests using the birds raised on the different diet regimes, results again suggested that diet consistency was important, as birds raised on a consistent diet were generally more aggressive than those raised on a variable diet, and also showed a trend towards winning more of their individual interactions. Consequently, in this study, nutritional stability made individuals generally more successful in conspecific interactions and thus more likely to hold a higher dominance position, though this is complicated by the fact that dominance was not linked to priority of access to other desirable resources such as potential mates. Using the same experimental design on the birds raised under the hormone treatment, there were trends to suggest that exposure to corticosterone could have negative effects on dominance-related behaviour under certain environmental conditions, but sample sizes were too small to allow firm conclusions.
Overall, this study provides strong evidence for the idea that behaviour can be modified by the environmental conditions an individual experiences in early life. Since behavioural traits will impinge significantly on individual fitness, such effects are of general interest. Since there are also studies that suggest that environmental shaping can be adaptive, I discuss the costs and benefits involved in the different behavioural phenotypes that arise as a result of environmental variation.